Electromagnetic radiation ranging from the infrared to the hard X-ray region is produced when electrons pass a series of magnetic fields of alternating polarity. Such an arrangement, termed a wiggler or undulator, consists of a series of opposed pairs of magnetic poles arranged to have alternate magnetic field direction between adjacent pole pairs. Charged particles such as electrons, moving in a magnetic field, experience a force perpendicular to both the magnetic field and velocity vectors. In response to the force, the electrons are accelerated in a transverse direction, and because of the alternating polarity of the magnetic poles, that direction is periodically reversed, causing the electrons to wiggle as they pass by the series of magnet poles. Such a typical wiggler, constructed using permanent magnet poles, is described on page 3607 of my article entitled "APPLICATION OF PERMANENT MAGNETS IN ACCELERATORS AND ELECTRON STORAGE RINGS" published in the Journal of Applied Physics, Volume 57, No. 8, Part IIA on Apr. 15, 1985. Prosnitz et al U.S. Pat. No. 4,506,229 also describes a Free Electron Laser using a transverse wiggler magnetic field.
The transverse acceleration of the electrons produdes electromagnetic radiation whose wavelength depends on the electron energy, the spacing between wiggler magnets, the magnetic field strength, and other factors. If the electron energy is extremely high, the radiation wavelength can be in the vacuum ultraviolet or even the X-ray region. The intensity of the light depends on the number of poles of the wiggler, so that longer and longer wigglers are being contemplated having as many as 2500 magnets. The energy of the emitted light is obtained at the expense of the energy of the electrons, resulting in a reduction in their mass, the reduction in electron velocity being negligible.
This reduction in mass is negligible unless the electron beam is bunched in such a way that the emitted radiation power is many orders of magnitude larger than it is for an unbunched beam. Under these circumstances, one talks of a Free Electron Laser or a transverse optical klystron.
When the reduction in electron mass is substantial, it is necessary to use a tapered wiggler, i.e , one in which the magnetic field strength of successive magnets is gradually decreased to maintain the relationship between wiggler period, magnetic field, and wavelength of emitted light. A tapered wiggler structure is shown, for example, in Madey et al U.S. Pat. No. 4,283,687.
While tapered wigglers are designed to adjust the field strength of the electromagnet poles of the undulator, it is necessary to accomplish this without causing steering or displacement problems.
Madey U.S. Pat. Nos. 4,461,004 and 4,479,219 disclose adjustment of the relative excitation of electromagnet poles in a free electron laser system using associated windings on magnet segments. Adler U.S. Pat. No. 3,300,728 discusses changing of the field strength of electromagnets in a cyclotron using aiding or bucking windings.